Fluidized beds are well suited for combustion of solid, liquid as well as gaseous fuels. The air necessary for the combustion is also used for fluidization of the bed material. The great thermal inertia of the bed material causes the combustion to take place at low temperature, which suppresses the formation of undesired nitrogen oxides. The great heat capacity reduces disturbances caused by variations in the fuel. If sulphur-containing fuels are burnt, additions of sulphur absorbents such as lime, limestone or dolomite to the bed material may cause the emissions of sulphur oxides to be greatly reduced, because the sulphur, which is released during the combustion, is bound to the sulphur absorbent.
In order for the combustion to take place in a fluidized bed, however, the bed material must first be heated to the ignition temperature of the fuel. Heating can be performed by heat exchanger tubes provided in the fluidized bed, through which tubes superheated steam from an external source is injected. Special combustors may be inserted into the fluidized bed. Starting fuel, with a low ignition temperature, may be supplied to the fluidized bed, be ignited and during combustion preheat the fluidized bed. In case of large fluidized beds, however, the storage of the starting fuel/air mixture in the fluized bed entails a risk of explosion.
The bed material can be preheated outside the bed vessel and be supplied in hot state, or be recirculated through an external heating agent.
Normally, however, preheating is performed by injecting hot gases, preferably flue gases, through the bed. The flue gases are obtained during combustion in special start-up combustion chambers or start-up combustors, which may be integrated into the gas paths (according to VDI-Bericht Nr. 322, 1978, pp. 139-145), or be free-standing. To avoid corrosion, associated with flue gas condensate on the cold walls of the gas paths, the gas paths are normally preheated with hot, preferably dry air of a temperature which should amount to about 250.degree. C., before combustion with fossil fuels is allowed in the gas paths.
Start-up combustors and start-up combustion chambers of a conventional design lead to heavy pressure drops and losses connected therewith across these combustors or combustion chambers if they are allowed to remain in the gas paths during operating conditions. To avoid this, the start-up combustors or start-up combustion chambers may either be introduced into or, by diverting the air flow to the fluidized bed, be connected to the gas paths only during start-up and be removed and disconnected from the gas paths when the plant is adjusted to operating condition.